Predicting and inhibiting aerodynamically generated noise for fast moving vehicles such as cars, aircraft and trains is increasingly important. The tonal noise generated by the shear-layer instability of air flowing around the cavity opening is especially one of the most significant and most intense sources of aerodynamically generated noise. Computational aeroacoustics (CAA) based on the CFD simulations of compressible Navier-Stokes equations offers the most general approach to predicting those aerodynamically induced sounds. Aeroacoustics is practically always associated with turbulent flow and turbulence is the major challenge for CFD simulations. Four different turbulence modelling approaches are examined in this work. Three of them belong to the LES method category and one uses the URANS approach. Appropriate numerical discretization and iteration schemes have been identified for each of these approaches and implemented in the OpenFOAM open source CFD platform. The accuracy, computational performance and convergence reliability of those schemes have been subsequently studied during three-dimensional CFD simulations on a model of a suitable real object. The CFD simulation results are validated by a measurement. An organ pipe has been chosen as the object of this CAA research because it uses self-sustained oscillations, commonly referred as shear-layer (Rossiter) modes, as the source of its tone generation. The numerical simulation of the shear layer modes, respectively the noise generated by instability in the shear layer, is the subject of this work.

A new parametric model for the growth rate of turbulent mixing layers is proposed. A database of experimental and numerical mixing layer studies was extracted from the literature to support this effort. The domain of the model was limited to planar, spatial, nonreacting, free shear layers that were not affected by artificial mixing enhancement techniques. The model is split into two parts which were each tuned to optimally fit the database; equations for an incompressible growth rate were derived from the error function velocity profile, and a function for a compressibility factor was generalized from existing theory on the convective Mach number. The compressible model is supported by a detailed evaluation of the currently accepted models and practices, including error analysis of the convective Mach number derivation and a critical analysis of Slessor’s re-normalization technique which affected his 1998 compressibility parameter. Analysis of the database suggested that a distinction should be made between thickness definitions that are based on the velocity profile and those based on the density profile. Additionally, the accumulation of different normalization approaches throughout the literature was shown to have introduced non-physical variance in the trends. Resolution of this issue through a consistent normalization process has greatly improved the normality and scatter of the data and the goodness-of-fit of the models, resulting in R2 = 0.9856 for the incompressible model and R2 = 0.9004 for the compressible model.

No description available.

Yes / This review paper addresses the structure of the mean flow and key turbulence quantities in free-surface flows with emergent vegetation. Emergent vegetation in open channel flow affects turbulence, flow patterns, flow resistance, sediment transport, and morphological changes. The last 15 years have witnessed significant advances in field, laboratory, and numerical investigations of turbulent flows within reaches of different types of emergent vegetation, such as rigid stems, flexible stems, with foliage or without foliage, and combinations of these. The influence of stem diameter, volume fraction, frontal area of stems, staggered and non-staggered arrangements of stems, and arrangement of stems in patches on mean flow and turbulence has been quantified in different research contexts using different instrumentation and numerical strategies. In this paper, a summary of key findings on emergent vegetation flows is offered, with particular emphasis on: (1) vertical structure of flow field, (2) velocity distribution, 2nd order moments, and distribution of turbulent kinetic energy (TKE) in horizontal plane, (3) horizontal structures which includes wake and shear flows and, (4) drag effect of emergent vegetation on the flow. It can be concluded that the drag coefficient of an emergent vegetation patch is proportional to the solid volume fraction and average drag of an individual vegetation stem is a linear function of the stem Reynolds number. The distribution of TKE in a horizontal plane demonstrates that the production of TKE is mostly associated with vortex shedding from individual stems. Production and dissipation of TKE are not in equilibrium, resulting in strong fluxes of TKE directed outward the near wake of each stem. In addition to Kelvin–Helmholtz and von Kármán vortices, the ejections and sweeps have profound influence on sediment dynamics in the emergent vegetated flows.

This study determined a superior data processing technique for correlating an acoustic signal passing through a subsonic jet engine exhaust in order to estimate the traversal time of the signal. Thrust measurement is possible with enough time delay estimates across different portions of the exhaust. This preliminary study did not take the full array of data necessary to measure thrust, but did validate key aspects of the measurement process. The turbulent shear layers of the exhaust spectrally broaden the signal, creating the appearance of spectral "haystacks", making traditional correlation methods unworkable. An experiment was performed to evaluate the ability of a novel sound source to produce a signal from which a reliable and precise time delay estimate could be found. The test apparatus was installed on either side of a Honeywell TFE731-2 turbofan research engine exhaust cone, with the source and receivers placed near the jet exit plane. The signal was then directed across the jet exhaust. This flow environment is considered an extreme challenge for accurate acoustic signal propagation. A key contribution of this paper is the determination that the Phase Transform processor of the Generalized Cross-Correlation (GCC) method produces the most reliable time delay estimates, for the given signal and flow conditions. Several alternative time delay estimators and GCC processors were examined and evaluated on this data. A proposed explanation is provided for why this time delay estimation technique produces the most accurate results, as well as explanations for why the technique became less reliable as the flow environment became more challenging, with an observed 22% anomalous TDE selection rate for the N1Corr = 60% and N1Corr = 70% conditions combined, versus only 6% for the idle and N1Corr = 50% conditions combined. This paper also details the development and first use of a novel acoustic source that produces a two-tone narrowband signal emanating from a single point – the dual Hartmann generator. / Master of Science / This study builds on a Computational Tomography (CT) technique that uses an acoustic signal and an array of receivers to measure the velocity and temperature of a gas flow field. In particular, the velocity and temperature field tested involves multiple turbulent and disruptive elements, requiring a loud and specifically designed signal. As such, a novel acoustic signal generator, the dual Hartmann generator, was designed that is both loud and produces a specific two-toned signal. The key contribution of the study was to process the data, comparing the sets of transmitted and received signals, in order to estimate the time delay amongst receiver pairs – a key input in the CT method. Traditional cross-correlation methods were inadequate, and multiple alternatives were evaluated. The Phase Transform (PHAT) technique showed the most promise, and an explanation is given for why this technique is most suitable for this type of signal.

No description available.

Separating and reattaching flows are important in a large number of engineering configurations. The flow through a sudden expansion (backward-facing step) represents a conceptually simple case of this class of flows and hence has been the subject of numerous studies. The present study focuses on the effect of the expansion ratio (defined as the ratio of downstream channel height to upstream channel height) on the unsteady flow features in the reattachment region and further downstream. It is known that this flow demonstrates two different instabilities; the Kelvin-Helmholtz shear layer instability, which scales with the shear layer thickness, and the instability associated with the separation bubble, which scales with the step height and has similarities to K´arman vortex shedding behind a cylinder.In addition to these, there is also a possibility of the presence of the ‘preferred’ mode of the jet issuing from the inlet channel of the sudden expansion, especially at high expansion ratios, where the flow resembles a wall jet. The aim of this study is to investigate experimentally the changes in the instability of the separation bubble, as the expansion ratio is changed, and its possible interactions with the other instabilities in the flow.One might expect some changes in the flow with expansion ratio, as at low expansion ratios, the configuration represents a simple backward-facing step geometry, while at high expansion ratios, the geometry approaches that of a wall jet. A variable expansion ratio backward-facing step facility has been developed in an open circuit wind tunnel.This facility permits continuous variation of the expansion ratio from 1 to around 6. Attention is focused on the turbulent regime of the flow, where the flow structure has been found in previous studies to be relatively insensitive to the Reynolds number. The inlet conditions have been kept constant with a thin turbulent boundary layer at the step, the boundary layer thickness at separation being approximately 14 % of the inlet channel height. The Reynolds number based on the inlet channel height, H, is kept constant at Re=48,000 and the expansion ratio is varied by changing the channel height downstream of the step. Detailed hot wire measurements have been made to characterize the spatial variation of the dominant frequencies in the flow at different expansion ratios. The expansion ratio has been varied from a low value of 1.14 to a high value of 3.25, and detailed measurements are obtained for five expansion ratios of 1.14, 1.3, 1.5, 2.0 and 3.0. Further, to elucidate the dominant vortical structures in the flow, Particle Image Velocimetry measurements have been undertaken simultaneously with hot wire measurements for the case of expansion ratio 1.5, which have permitted the conditional averaging of vorticity fields.These investigations have brought forth some interesting features of the flow over a backward-facing step. Results for the time-mean properties of the flow indicate that the shear layer separating from the step deviates from a free mixing layer behaviour away from the step, possibly due to its interaction with the wall and the recirculation region underneath it. At any given streamwise location, the shear layer momentum thickness, θ, is seen to increase with the expansion ratio. Further, upto reattachment, the momentum thickness of the shear layer is seen to scale with the step height, h, independent of its initial thickness at separation, θo, as long as the boundary layer at separation is sufficiently thin as compared to the step height. Investigations for the unsteady flow features show that the frequency of the dominant peak in the velocity spectrum, supposed to represent the passage frequency (Strouhal number, S, based on the step height, h, and the inlet velocity, U) of the vortical structures, varies in the cross stream (y) direction, in addition to its expected variation in the streamwise (x) direction. The variation of the Strouhal number in the cross stream direction is seen to scale with the local momentum thickness of the shear layer, except for locations very close to the step. To characterize the development of the dominant frequency in the streamwise direction, the maximum value of the Strouhal number at a streamwise location is taken to be the representative value for that streamwise location. The Strouhal number is seen to decrease in the streamwise direction, from a very high value near the step, to a value of approximately 0.08 in the reattachment region, and remains constant further downstream. This value, supposed to represent the large scale structures shed from the reattachment region, is seen to remain very close to 0.08 for all Expansion ratios investigated. Conditional averaging of the vorticity fields in the reattachment region is done for an expansion ratio of 1.5, to get a detailed picture of the unsteady flow field. The hot wire signal at the outer edge of the shear layer in the reattachment region, which represents the non-dimensional structure passage frequency of S=0.08, is used as the conditioning signal. Results seem to indicate that the recirculation region, or the ‘bubble’ divides into two cells, and sheds the downstream cell quasi-periodically. The passage of these structures through the reattachment region seems to be concomitant With a local vertical motion of the shear layer. Further, the streamwise development of the local Strouhal number, Sθ, based on the local momentum thickness of the shear layer, and the local free stream velocity, Umax, indicates a possibility of a coupling between the shear layer and the structures shed from the reattachment region.

No description available.

A robotically-controlled actuation system has been developed and built to perform active open-loop flow control experiments on transitional and turbulent backward-facing step flows in water. Control of the reattaching shear layer used hydraulic suction-and-blowing actuation emanating from 128 individual ports along the separation edge of the step. Each ports perturbation was periodic in time, but individually controlled to produce either spanwise-invariant (2D) or spanwise-varying (3D) spatial actuation profiles. An image processing system and special aqueous tuft were developed to measure the length of the recirculation bubble. Multiple images of a tuft array were time-averaged to do so. In general, 3D forcing was no more effective in reducing bubble length than 2D forcing. However, greater local spanwise reductions in reattachment length were observed for some cases of spanwise-varying forcing. Backlit dye was used to track the evolution of vorticity in the flow in video and still images.

Les décollements turbulents massifs sont des phénomènes communs qui peuvent causer des pertes et de nuisances aérodynamiques importantes dans les écoulements industriels, par exemple à l’arrière d’une aile d’avion. Ce travail contribue à leur compréhension par l’analyse phénoménologique d’un décollement turbulent, représentatif d’un grand nombre d’écoulements réels. Le premier objectif est d’identifier les lois d’échelle des décollements turbulents, notamment en rapport avec les caractéristiques de l’écoulement à l’amont de la rampe. Un deuxième objectif est l’analyse, à grande et à petite échelle, des mécanismes de transport de fluide qui pilotent le fonctionnement des décollements. A cet effet, une approche originale est proposée, basée sur une description expérimentale et analytique de la couche cisaillée décollée et des interfaces turbulentes qui la délimitent. Nos résultats suggèrent que les lois d’échelle du décollement varient de façon complexe selon l’interaction de la couche limite à l’amont, de la couche cisaillée et de l’écoulement potentiel extérieur. La taille du décollement est liée à l’intensité de l’entraînement turbulent de masse dans la couche cisaillée, qui à son tour dépend de la turbulence dans la couche limite, bien à l’amont du point de décollement. Cette dépendance pourrait s’appliquer à toute la gamme d’échelles turbulentes responsables du transport de masse. Ces observations montrent clairement le rôle de la couche cisaillée dans le fonctionnement des décollements massifs et suggèrent la faisabilité de stratégies de contrôle nouvelles, de type retro-action ou prédictif, basée sur l’entrainement turbulent. / Massive turbulent separations are common phenomena that can cause sizeable aerodynamical losses and detrimental effects in industrial flows, for example on airplane wings. This work contributes to their understanding with a phenomenological analysis of a canonical turbulent separation, representative of a large number of real flows. The first objective is to identify the scaling laws of turbulent separations, in particular with respect to their dependencies on the characteristics of the flow upstream of the ramp. A second objective is the analysis, both at large and small scale, of the transfert mechanisms that drive the functioning of separated flows. To this end, a new approach is proposed, centered on the experimental and analytical description of the separated shear layer and of the turbulent interfaces that bound it. Our results suggest that the scaling laws of the separated flow vary in a complex way, in function of the interaction of the incoming boundary layer, the separated shear layer and the free-stream. The size of the separation is related to the intensity of turbulent mass entrainment within the shear layer, which in turn depends on the turbulence in the incoming boundary layer, well upstream of the separation point. This dependency might apply over the entire range of turbulent length scales that are responsible for mass transfer. These observations clearly show the role of the shear layer in the functioning of massive separation. They also suggest the feasibility of new control strategies, both of feedback and feed-forward type, based on turbulent entrainment.